1. Field of the Invention
This invention relates to the synthesis of hydrocarbons from a methane source. A particular application of this invention is a method for converting natural gas to more readily transportable material using a methane conversion catalyst formed using a reducible metal oxide and a molten salt.
2. Description of the Pertinent Art
A major source of methane is natural gas. Other sources of methane have been considered for fuel supply (e.g., the methane present in coal deposits or formed during mining operations). Relatively small amounts of methane are also produced in various petroleum processes.
The composition of natural gas at the wellhead varies, but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range of about 40 to about 95 volume percent. Other constituents of natural gas include ethane, propane, butane, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium, and nitrogen.
Natural gas is classified as dry or wet, depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons, although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas; processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Large-scale use of natural gas often requires a sophisticated and extensive pipeline system. Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting and revaporizing natural gas are complex, energy intensive and require extensive safety precautions. Transport of natural gas has been a continuing problem in the exploitation of natural gas resources. It would be extremely valuable to be able to convert methane (e.g., natural gas) to more readily transportable products. Moreover, direct conversion to olefins such as ethylene or propylene would be extremely valuable to the chemical industry.
Recently, it has been discovered that methane may be converted to higher hydrocarbons by a process which comprises contacting methane with an oxidative synthesizing agent at synthesizing conditions (e.g., at a temperature selected within the range of about 500.degree. to about 1000.degree. C.). An oxidative synthesizing agent is a composition having as a principal component at least one oxide of at least one metal, which composition produces higher C.sub.2 + hydrocarbon products, water and a composition comprising a reduced metal oxide when contacted with methane at synthesizing conditions. Reducible oxides of several metals have been identified which are capable of converting methane to higher hydrocarbons. In particular, oxides of manganese, tin, indium, germanium, lead, antimony, praseodymium, and bismuth are most useful.
Typically, the oxidative synthesizing agent is in fluidized bed reactors and in packed bed cycling operations.
In the fluidized bed converters, separated fluidized bed vessels are used for reaction and regeneration; the agent particles are fluidized by the methane feed gas in a reactor and by the oxidizing gas (e.g., air) in a regenerator, with the agent transferring continuously between the two vessels. This process has the advantages of steady-state operation and good temperature control, facilitating well controlled product compositions and per pass conversions. However, the motion in the fluidized bed causes the agent particles to break up into fines by attrition; the fines then elute out of the vessels with the effluent gases, thereby reducing the service life of the agents. The fluidized particles are also abrasive, and tend to erode the internals of the vessels and piping. Extra flue gas treatment steps may also be needed to control emission of entrained particulates.
In the packed bed converters, the agent is packed into a single vessel where it is sequentially and cyclically reduced (reaction with the methane feed), regenerated (oxidized with air or oxygen), and cooled with gas to remove the heat released during regeneration from the agent particles. Cycle times of less than 5-10 minutes are typical. The cyclic packed bed converter is inherently nonsteady state, with both agent temperatures and bound oxygen concentrations varying with both time and position in the bed. Consequently, during the reductive reaction step, the product distribution, local bed temperature, and conversion of the feed varies over time. Thus, this type of converter is difficult to control and is subject to severe mechanical stress due to rapid thermal cycling.
In an article entitled "Partial Oxidation of O-xylene in Melts containing Vanadium Pentoxide" by Satterfield and Loftus, the authors disclose contacting methods for catalyzed gas-phase reactions to include bubbling reactants through a melt which acts as an oxidizing agent or as a catalyst. The reactants would be oxidized by the melt in one reactor, and the reduced melt reoxidized by a second gas stream such as air in a second reactor, and then recirculated to the first reactor.
In U.S. Pat. No. 4,107,280, there is disclosed a process for oxidizing hydrogen halide by means of a catalytically active molten salt. The molten salt is comprised chiefly of alkali metal pyrosulfates and sulfates, and a lesser amount of vanadium pentoxide which serves as a source of oxygen for reaction with the hydrogen halide. The gaseous effluent stream includes halogen, steam, and unreacted hydrogen halide, but no free oxygen. The molten salt is transported to a second reactor where it is contacted with an oxygen-bearing gas such as air. Other soluble metal oxides may be used for the oxidation of hydrogen chloride, including copper, iron, chromium or manganese. Less suitable metal oxides include lead, nickel, cobalt or uranium.
Accordingly, an object of this invention is to provide an improved process for converting methane to higher hydrocarbons. A further object of this invention is an improved method of contacting the methane with an oxidative synthesizing agent while effectively transferring heat to the methane feed and providing a uniform thermal atmosphere.
Other aspects, objects and the several advantages of this invention will become apparent to those skilled in the art upon reading this Specification and the appended claims.